Load monitoring of large and expensive structural frames, such as aircraft and roadway bridges has been used to measure loads that lead to structural fatigue. Such measurements have identified structures in need of maintenance. They have also provided a way to lower overall maintenance costs, increase life span, and delay replacement costs.
In one example, the US Navy used conventional bonded foil type strain gauges to track operational structural loads on its fleet of fixed wing aircraft, including the F-18. The hard-wired structural monitoring system provided load information that greatly increased the operational life of the Navy's F-18 fleet. On average, these aircraft now operate for over 13,000 flight hours, more than twice the manufacturer's design life estimate of 6000 hours. If these flight hours had been replaced with new aircraft, as originally planned, the cost could be estimated at about $11B. However, this monitoring scheme has been costly and difficult to maintain for application to other fleets of aircraft.
Helicopter component loads, for example, have traditionally been monitored on only one or two strain gauge instrumented flight test aircraft, using slip rings to provide data on rotating components. These techniques were too costly and difficult to maintain for use on an entire fleet of helicopters. Instead of basing fatigue life of critical components, such as the pitch links, pitch horns, swash plate, yoke and rotor, on measured loads on those components on each helicopter, fatigue life was conservatively estimated based on the aircraft's flight hours, as described in an article, “The Art of Helicopter Usage Spectrum Dev.” by S. Moon & C. Simmerman, Am. Helicopter Soc. (AHS) 61st Annual Forum, Grapevine, TX June 1-3, 2005.
Improvements, such as implementing energy harvesting, combined with advanced, micro-power wireless sensing electronics, enabled the realization of direct tracking of the operational loads on these rotating structures, resulting in improved condition based maintenance and enhance safety based on data obtained in instrumented test aircraft, as described in the articles, “Energy Harvesting Wireless Sensors for Helicopter Damage Tracking” by Arms et al., AHS 62, Phoenix, AZ, May 11th, 2006 and “US Navy Roadmap to Structural Health and Usage Monitoring” by Maley at al., AHS 63, Virginia Beach, VA, May 1-3, 2007.
In a report published by the Finnish Air Force, “A Review of Aeronautical Fatigue Investigations in Finland During the Period Feb 2001 to March 2003” by Siljander, A., Finnish Air Force Headquarters, Aircraft & Weapons Systems Division, Research Report BTUO33-031123, Presented at 28th Conference of the International Committee on Aeronautical Fatigue, Lucerne, Switzerland, May 2003, the authors stated that a reliable strain gauge fatigue damage detection system requires the following items:                an understanding of the monitored structures' mechanical behavior due to operational and environmental loads;        proper placement of sensors in the vicinity of structural fatigue “hot spots”;        proper tuning of the data acquisition parameters, such as sampling resolution and sample rates;        tailored algorithms to analyze the sensor data to come up with reliable indications of structural deterioration, which would trigger maintenance actions.        
In order to obtain useful loads data from an airframe structure instrumented with strain gauges, the strain gauges are calibrated to relate the strain gauge data to the loads experienced by the aircraft. This can be done on one or two flight test vehicles by applying static loads and moments in a full-scale test rig and monitoring the responses from various strain gauges. For a fleet of aircraft, however, this approach has been too expensive and time consuming.
Flight calibration has been a practical alternate approach, in which each aircraft in the fleet is flown in specific, proscribed maneuvers to create a known loading condition, and this in turn is used to calibrate the strain gauge's output. However, these calibration methods introduce variability based on the pilot's technique as well as variation due to strain gauge installation and manufacturing variations. Furthermore, portions of the airframe such as the vertical tail and canopy sill are not easily loaded through proscribed flight maneuvers.
Similar monitoring has been needed for road and railway bridges across the United States, as described in US-DOT, Federal Highway Administration, http://www.tfhrc.gov/pubrds/marapr01/bridge.htm, by Phares, Brent M., Rolander, Dennis D., Graybeal, Benjamin A., and Washer, Glenn A., March/April 2001 and in “Structural Health Monitoring of Bridges for Improving Transportation Security,” by Catbas, F. Necati; Susoy, Melih; and Kapucu, Naim Journal of Homeland Security and Emergency Management: Vol. 3: Iss. 4, Article 13 (2006), http://www.bepress.com/jhsem/vol3/iss4/13.
Recent catastrophic failures of bridges have highlighted the need for accurate loading information that would trigger maintenance activities to avoid the tremendous loss of life and property that accompany these events.
Construction of an instrumented shear pin for a load cell is described in U.S. Pat. Nos. 3,695,096 (“the '096 patent”) and 4,283,941 to Kutsay, both incorporated herein by reference. The '096 patent provides a pin or bolt with an axial bore that contains electrical strain gauges attached to its circumferential inner wall and having leads connected to exterior instrumentation, such as Wheatstone bridge instrumentation. The arrangement and orientation of the gages permit evaluation of the applied load both as to magnitude and direction.
A process for inserting the strain gauges into the axial bore in a bolt is described in U.S. Pat. No. 2,873,341 (“the '341 patent”) to Kutsay, incorporated herein by reference. In the '341 process, a core with the strain gauge mounted thereon is dipped or coated with epoxy and pressed into the bore. In one embodiment described in the '341 patent, a metallic core is cooled in dry ice or in any other manner and then inserted in the bore and allowed to expand therein so that the strain gauge is squeezed against the inner wall of the bore to make a shrink fit.
International patent publication WO 2006/067442 to El-Bakry, et al, (“the El-Bakrey patent application”) incorporated herein by reference, describes a pin bearing arrangement for use on an aircraft landing gear that includes a pin, and means including strain gauges for measuring shear loads and accelerometers for measuring loads sustained by the pin. Inside the pin there is provided a processing unit, a memory store, and a battery-based power source. During normal operation of the aircraft fatigue loads can be monitored, the processing unit receiving input signals from the means for measuring loads and storing load data in the memory. The stored data is periodically extracted from the memory during maintenance of the aircraft. Thus, the pin bearing arrangement is able to perform the function of a self-contained load data logging device for logging data concerning loads sustained by the pin. And there is no need for any part of the pin bearing arrangement (in particular, the means for measuring loads) to be connected to any part of the aircraft's standard computer system.
A better system for monitoring structures and reducing failures has been needed that does not rely on expensive monitoring and calibration schemes. Several embodiments of such a system are provided in this patent application.